INTRODUCTORY, 5 



any mecans necessary that a distinct nervous system should 

 be present. 



As a general rule, animals can not only change their 

 place, but they are also enabled to obtain some knowledge 

 of the nature and characters of bodies outside themselves, 

 by means of certain " organs of the senses." Thus, the 

 higher animals have special organs for the perception of 

 light, sound, odours, and tastes or flavours, whilst their 

 sense of touch enables them to detect the more material 

 properties of bodies foreign to themselves, as well as of 

 their own bodies. As a general rule, these senses, as in 

 the case of the locomotive organs, are vnider the control of 

 a "nervous system;" but there are many animals in which 

 a nervous system is absent, and to which we have, never- 

 theless, no reason to deny the possession of at any rate 

 some of the senses, if in only a rudimentary form. 



For the preservation and continued existence of each 

 individual animal it is only necessary that it should be 

 able to nourish itself, and that it should have certain 

 relations wdtli the outer world. Under any circumstances, 

 however, the life of each individual animal comes to an 

 end some time or other, owing to the inevitable failure of 

 the nutritive powers which comes on when the animal has 

 reached a certain age. In other words, death comes sooner 

 or later to each individual animal, in consequence of its 

 reaching a period of its life when it is unable to " assim- 

 ilate" food enough to replace the constant loss of tissue 

 caused by living. In every kind of animal, therefore, the 

 individual dies after a longer or shorter period of exist- 

 ence, but the hind or "species" of animal does not die or 

 disappear, because animals are endowed with the power 

 of reproducing their like. Thus, by producing young like 

 themselves, all animals have the power of continuing their 

 kind through successive generations, in spite of the fact 

 that the individuals of each generation perish, each in his 

 appointed time. 



It follows from what has been here remarked, that an 

 animal, in its life, discharges three principal sets of func- 

 tions. It nourishes itself; it has certain relations with 



